In recent years, with the advent of the personal wireless communication techniques, wireless communication devices are becoming more important in everyday life. In wireless communication applications, devices, such as double balanced mixers, push-pull amplifiers, balanced frequency multipliers, phase shifters, dipole antenna feeds, balanced modulators, 180-degree hybrids and the like, employ a balanced-to-unbalanced converter (or sometimes called “balun” for short), which converts an unbalanced signal to a pair of balanced differential signals with equal amplitude and anti-phase, thereby reducing common-mode noise with the transmission of the differential signals.
However, a traditional balun is generally consisted of high-pass filters and low-pass filters. Therefore, there is a limitation that both the unbalanced input end and the balanced output end have to be in real impedance. However, more and more wireless communication chips require the use of differential signals with complex impedance as input signals. Thus, there is a need for a balun that is capable of converting an unbalanced signal with real impedance into a balanced signal with complex impedance.